Audi 5000S Quattro not charging

[Dave C]

Are you using Audi specific belts? The belts from places like NAPA do not
have the correct profile ( V-shape) and do not sit in the pulley correctly.
If you have the wrong belt this could be your problem.

The next thing I would suspect is worn brushes on the alternator voltage
regulator. If your car is a turbo that's too bad because I think you will
need to remove the alternator to look at the brushes. If it's non-turbo I
think you have much better access to remove the voltage regulator (VR) and
check the brushes without removing the alternator.
- -
Dave C.